Transformation and Microcrack Toughening as Complementary Processes in ZrO 2 ‐Toughened Al 2 O 3

Both tetragonal ( t ) and monoclinic ( m ) ZrO 2 particles in ZrO 2 ‐toughened Al 2 O 3 can give rise to toughening. In the stress field of propagating cracks, the t ‐ZrO 2 particles can undergo the stress‐induced t → m transformation, and the residual stresses around already‐transformed m ‐ZrO 2 particles can cause microcracking. The t ‐ZrO 2 particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr0 2 ‐toughened Al 2 O 3 's should contain a mixture of t ‐ and m ‐ZrO 2 .